Diffraction grating. The principle of operation of the diffraction grating
It is no secret that, along with tangible matter, we are surrounded by wave fields with their own processes and laws. It can be electromagnetic, and sound, and light vibrations, which are inextricably linked with the visible world, interact with it and influence it. Such processes and effects have long been studied by various scientists who derived the basic laws that are relevant to this day. One of the widely used forms of interaction between matter and wave is diffraction, the study of which led to the emergence of such a device as a diffraction grating, which has been widely used in devices for further investigation of wave radiation, and in everyday life.
Diffraction refers to the process of bending around light, sound and other waves of an obstacle encountered on their way. More generally, this term can be called any deviation of wave propagation from the laws of geometric optics, occurring near obstacles.Due to the phenomenon of diffraction, the waves fall into the region of a geometric shadow, bend around obstacles, penetrate small holes in screens and so on. For example, you can well hear the sound, being around the corner of the house, as a result of the fact that the sound wave bends around it. The diffraction of light rays is manifested in the fact that the area of the shadow does not correspond to the throughput hole or the existing obstacle. The principle of operation of the diffraction grating is based on this phenomenon. Therefore, the study of these concepts is inseparable from each other.
The concept of a diffraction grating
The diffraction grating is an optical product, which is a periodic structure consisting of a large number of very narrow slits separated by opaque gaps.
Another variant of this device is a set of parallel microscopic strokes, having the same shape, applied on a concave or flat optical surface with the same predetermined pitch. When light waves are incident on the grating, the process of redistribution of the wave front in space occurs, which is caused by the diffraction phenomenon.That is, white light is decomposed into separate waves having different lengths, which depends on the spectral characteristics of the diffraction grating. Most often for work with the visible range of the spectrum (with a wavelength of 390-780 nm) use devices having from 300 to 1600 strokes per millimeter. In practice, the lattice looks like a flat glass or metal surface with rough grooves (strokes) applied with a certain interval, which do not transmit light. With the help of glass gratings, observations are conducted both in transmitted and reflected light, with the help of metal gratings - only in reflected light.
Types of grids
As already mentioned, diffraction gratings are reflective and transparent in terms of the material used in the manufacture and characteristics of their use. The first are devices that represent a metallic mirror surface with applied strokes, which are used for observations in reflected light. In transparent gratings, strokes are applied to a special optical, ray-transmitting surface (flat or concave), or narrow slits are cut in an opaque material.Research in the use of such devices is carried out in transmitted light. An example of a coarse diffraction grating in nature is eyelashes. Looking through the narrowed eyelids, it is possible at some point to see the spectral lines.
The work of the diffraction grating is based on the phenomenon of diffraction of a light wave, which, passing through a system of transparent and opaque areas, is divided into separate beams of coherent light. They undergo diffraction on the strokes. And at the same time interfere with each other. Each wavelength has its own diffraction angle; therefore, white light is decomposed into a spectrum.
Resolution of the diffraction grating
Being an optical device used in spectral devices, it has a number of characteristics that determine its use. One of these properties is the resolution, which consists in the possibility of separate observation of two spectral lines with a close wavelength. Enhancement of this characteristic is achieved by increasing the total number of strokes present in the diffraction grating.
In a good device, the number of strokes per millimeter reaches 500, that is, with a total lattice length of 100 millimeters, the total number of strokes will be 50,000. This figure will help to achieve narrower interference maxima, which will allow you to select close spectral lines.
Use of diffraction gratings
Using this optical device, you can accurately determine the wavelength, so it is used as a dispersing element in spectral instruments for various purposes. The diffraction grating is used to highlight monochromatic light (in monochromators, spectrophotometers, etc.), as an optical sensor of linear or angular displacements (the so-called measuring grating), in polarizers and optical filters, as a divider of radiation beams in the interferometer, as well as in antiglare glasses .
In everyday life, you can often come across examples of diffraction gratings. The simplest of the reflective can be considered cutting of compact discs, since a track with a 1.6 μm step between the turns is spirally applied on their surface. The third part of the width (0.5 μm) of such a track falls on a depression (where the recorded information is contained) that scatters incident light,and about two thirds (1.1 microns) is occupied by an intact substrate capable of reflecting rays. Therefore, the compact disc is a reflective diffraction grating with a period of 1.6 μm. Another example of such a device are holograms of various types and directions of application.
To obtain a high-quality diffraction grating, it is necessary to observe very high precision manufacturing. Error when applying at least one stroke or slit leads to an instant culling of the product. For the manufacturing process, a special dividing machine with diamond cutters is used, attached to a special massive foundation. Before the beginning of the process of cutting the lattice, this equipment should work for 5 to 20 hours in idle mode in order to stabilize all the nodes. The fabrication of one diffraction grating takes almost 7 days. Despite the fact that the application of each stroke occurs in just 3 seconds. The gratings in this production have parallel strokes equally distant from each other, the shape of which depends on the profile of the diamond cutter.
Modern diffraction gratings for spectral instruments
Nowadays, a new technology of their manufacture has spread, with the help of education on special photosensitive materials, called photoresists, of the interference pattern obtained from laser radiation. As a result, products with a holographic effect are manufactured. Strokes can be applied in a similar way on a flat surface, obtaining a flat diffraction grating or a concave spherical, which will give a concave device having a focusing effect. In the design of modern spectral instruments used both.
Thus, the phenomenon of diffraction is common in everyday life everywhere. This leads to the widespread use of a device based on this process, such as a diffraction grating. It can either become a part of research equipment, or meet in everyday life, for example, as the basis of holographic products.